CAPE CANAVERAL, Florida Voyager 1, launched in 1977 to explore the outer planets, has passed into a new region on its way out of the solar system, scientists said on Wednesday.
The spacecraft, now more than 11 billion miles (18 billion km) away, detected two distinct and related changes in its environment on August 25, 2012, scientists write in paper to be published in Geophysical Research Letters and emailed to Reuters on Wednesday.
The probe detected dramatic changes in the levels of two types of radiation, one that stays inside the solar system, the other which comes from interstellar space.
The number of particles inside the solar system's bubble in space, a region called the heliosphere, dropped to less than 1 percent of previously detected levels, while radiation from interstellar sources nearly doubled, said astronomer and lead author Bill Webber, professor emeritus at New Mexico State University in Las Cruces, New Mexico.
Scientists are not yet ready to say Voyager is in interstellar space, however.
The probe, which blasted off from Cape Canaveral, Florida, on September 5, 1977, may be in a new and previously unknown boundary region between the heliosphere and interstellar space.
Webber refers to this area as the "heliocliff."
"It's outside the normal heliosphere," Webber said in a statement. "Everything we're measuring is different and exciting."
In December, scientists said Voyager had reached what they called a "magnetic highway," where magnetic field lines from the sun connect with magnetic field lines from interstellar space.
"We believe this is the last leg of our journey to interstellar space," Voyager project scientist Edward Stone said at the time. "Our best guess is it's likely just a few months to a couple years away."
In a statement on Wednesday, Stone said more evidence is needed to indicate Voyager has left the solar system.
"It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," Stone said.
"A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed," he said.
Voyager 1 and a sister spacecraft, Voyager 2, were launched 16 days apart in 1977 to fly past Jupiter, Saturn, Uranus and Neptune.
Voyager 2 is traveling on a different path out of the solar system and is not believed to have reached the magnetic highway toward interstellar space yet.
(Editing by Kevin Gray and Cynthia Osterman)